Transcript Science Earth`s Changing Surface

Science
Earth’s Changing
Surface
Chapter 4
Section 2
Finding the Relative Age of Rocks
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Relative means compared to others
The relative age of a rock does not provide
its absolute age
The absolute age of a rock is the # of yrs
since the rock formed
The Position of Rock Layers
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The Law of Superposition is used to
determine the relative ages of sedimentary
rock layers
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In horizontal sed rock layers, the oldest layer is
@ the bottom
Each higher layer is younger than the layers
below it.
Other Clues to Relative Age
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Extrusion is lava that hardens on the
surface
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Intrusion is where the magma cools &
hardens into a mass of igneous rock
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The rock layers below an e are always older
The i is always younger than the rock layers
around or beneath it
A fault is a break in E’s crust
Gaps in the Geological Record
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The surface where new rock layers meet a
much older rock surface beneath them is
called unconformity
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This is a gap in the geological record
Using Fossils to Date Rocks
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Index fossils help geologists match rock
layers
To be useful as an index, a ‘f’ must be
widely distributed & rep a type of org that
existed only briefly
I.F. are useful because they tell the relative
ages of the rock layers in which they occur